Many countries face high costs – economical, environmental and social – due to the presence of IAS. The financial and management costs associated with their eradication are astronomical and their proliferation affects the potential of countries to meet their development and environmental objectives. Resources spent on trying to control IAS could be redirected to other development initiatives, such as the implementation of the MDGs. This is an important reason to adopt approaches which control and prevent introductions.

Invasive alien species cost millions of US dollars annually in terms of lost revenue and expenditure on control measures. While the actual costs of IAS are unknown, they are believed to be substantial. The global economic costs of IAS are estimated by IUCN to be about US$400 000 million annually (UNEP 2003); IUCN also finds that IAS threaten the success of current and planned World Bank projects to the value of more than US$13 000 million (UNEP 2004).

Currently, Africa spends an estimated US$60 million annually on the control of IAS (CBD 2005). The African Ministerial Conference on the Environment (AMCEN) plans to raise a further US$265 million to fund various projects related to IAS in Africa over the next three to five years (UNEP 2004).

Figure 2: Estimated percentage invasion per tertiary catchment In South Africa, alien plant species now cover more than 10.1 million ha, threatening indigenous plants (ITC undated). (See Figure 2). Freshwater systems and the Cape Floral Kingdom (a global centre of biodiversity) are particularly at threat and the South African government therefore established the “Working for Water” programme. This programme seeks to remove IAS infestations from water catchment, and at the same time provide poverty relief (van Wilgen and others 1998). In South Africa, in addition to altering water-flow, IAS have had other important impacts on endemic biodiversity and ecosystem services. Nitrogen-fixing plants such as Acacia saligna alter the nitrogen cycle, impacting on native plants adapted to low nutrient conditions, such as, for example, many of the fynbos species. The costs associated with eradicating IAS, as shown in Box 6, have exceeded US 100 million.

In the Western Indian Ocean (WIO) countries, IAS pose a serious threat to forests and thus place biodiversity, including many endemics, at risk. Among these invasive woody species are Paraserianthes falcataria (Albizia), Adenanthera pavonia (Agati), Clidemia hirta (Creole name: Faux Watouk), Cinnamomum verum, Chrysobalanus icaco (Prune de France), Psidium cattleianum (wild guava), Syzygium jambos, Astonia macrophylla (Bois jaune) and Tabebuia pallida (Calice du pape) (IUCN/SSG/ISSG). In addition to the costs to biodiversity, governments incur substantial financial and management costs. In the Seychelles, for example, the Ministry of Environment is involved in a programme for the eradication of IAS, including those listed below, and the replanting of indigenous species; public education is seen as an important aspect of this (Ministry of Environment Seychelles, undated):

  • Clidemia hirta grows quickly, particularly in disturbed areas, and displaces native plants. It competes effectively for light and soil nutrients and is therefore a successful invasive.
  • Cinnamomum verum, introduced in the 1970s for economic reasons, has spread so rapidly that today it is the most widely distributed and probably the most numerous plant in the Seychelles.
  • Chrysobalanus icaco was originally introduced to prevent erosion on steep slopes. Dense thickets of this species have now become established on many steep erosion slopes. It is difficult to get rid of this species once it has become established. It also invades areas where the indigenous forest had been cleared.
  • Syzygium jambos (Jambrosa) tends to replace the naturally occurring vegetation, including forests in river ravines. Jambrosa is native to Indo-Malaysia and was recorded as being well established in the Seychelles as early as the 1870s. It is still planted by many people for its edible fruits.

Across Africa, IAS in the genus Striga have a direct impact on local livelihoods, affecting more than 100 million people and as much as 40 per cent of arable land in the savannahs. The cost of eradicating it is reportedly between US$7-13 000 million annually (UNEP 2004). These invasives stunt maize plant growth by attacking the roots and sucking nutrients and water (Ithula 2004) and thus in addition to the direct financial costs have implications for food security.

Box 6: Black wattle: weighing the costs-and-benefits

The black wattle tree has been used in Africa as a commercial plantation species for many years. It has a variety of important uses (IUCN/SSC/ISSG 2004):

  • Tannin compounds extracted from the bark are used in the production of soft leather.
  • Resins, thinners and adhesives, can be made from bark extracts.
  • The timber is used for building materials.
  • Charcoal produced from wood is used for fuel.
  • The pulp and woodchips are used to produce paper.

It also has some well known medical applications, including its use as an astringent. Plantings of wattle tress have also been used as a soil stabilizer to decrease erosion. The agroforestry industry promotes the use of the species (among other similar species) as a potential “soil improver.”

It is nevertheless a highly invasive species – it produces large amounts of long-lived seeds – and it competes with and replaces indigenous vegetation. It may replace grass communities, reducing the carrying capacity of the land (IUCN/SSC/ISSG 2004). By causing an increase in the height and biomass of vegetation, black wattle infestations increase rainfall interception and transpiration, which causes a decrease in stream-flow. The soil underneath becomes desiccated more quickly than it does under grass. Black wattle stands also destabilize stream banks and support a lower diversity of species.

In South Africa, authorities are fighting to combat black wattle, which was introduced about 150 years ago to provide bark products. The black wattle is one of about 110 IAS of almost 750 tree species and 8 000 shrubby and herbaceous species that were imported into South Africa from countries in North America, South and Central America, Australia, Europe, Oceania and Asia (Sweet 1999). It has been described as “the number one threat to biodiversity in the Cape Floral Kingdom” (de Bakker 2003). Of the remaining natural areas of the Cape region, 17-24 per cent have been invaded by acacias (Musil 1993). The Cape Floral Kingdom, a biodiversity hotspot at Africa’s southwesternmost tip, is of huge importance because it contains 1 per cent of the world’s total plant species as endemics (de Bakker 2003).

Efforts to eradicate black wattle have come at tremendous cost:

  • Since 1995, the financial cost of control comes to US$70 million (Preston 2004) and about 40 000 workers have been involved in removing the black wattle together with other invasives.
  • More than 5 000 million invasive alien trees, of which many are black wattle, have been removed since 1995 (de Bakker 2003).
  • It costs the South African government an estimated US$40 million annually for manual and chemical control of IAS in the Cape Floral Kingdom (IUCN 2001).

Sources: de Bakker 2003, IUCN 2001, IUCN/SSC/ISSG 2004, Musil 1993, Preston 2004


While islands may not be more susceptible to invasions by alien species than continental landmasses, they are, however, considered to be particularly vulnerable to the impacts of such invasions (CBD 2003 and IUCN/SSC/ISSG 2001). On islands, IAS are now on a par with habitat loss as the lead driver of species extinctions over the last 20 years (Baillie and others 2004).

Important opportunities exist for effective control for terrestrial IAS. These can be effectively controlled through customs and border monitoring; these measures have greater potential for success on islands than in countries that share boundaries (IUCN/SSC/ ISSG 2001 and Wittenberg and Cock 2001). In terms of responses on islands, research shows that the experience of one island country can be invaluable in managing IAS on another island even where there are major differences in climate and ecological systems. Key similarities such as the role and nature of trade may be significant. Areas where a cooperative initiative on island IAS may be especially valuable include (IUCN/SSG/ISSG 2001):

  • Undertaking the eradication of IAS;
  • Undertaking the management of IAS where eradication is not currently feasible, to low levels that allow recovery of biodiversity values;
  • Training and other capacity enhancement activities; and
  • Undertaking quarantine and contingency response activities to prevent the establishment of new populations of IAS that might threaten ecosystems or species (including the control of movement in the country).